We just bought a new (used) boat from a private party. It's a 1992 Cobia San Marino with a 4.3 V6 mercruiser I/O motor. This is our first boat. The first day we took it out everything was fine...but when we left, the bilge pump was still running after the boat was on the trailer. At the time I didn't know that they automatically turn on and off as needed so I disconnected the negative terminal on the battery. I connected the battery back up the next day and it was fine. So we took it out yesterday and my uncle drove it from the boat launch to his pier(about a mile). It took him an hour. He said the trim wouldn't work. After about 5-10 minutes I checked the engine compartment....didn't notice that there were two negative wires--I only reconnected one. So I connected it and the trim worked. Now today we took the boat out and everything worked fine-until we got out to the middle of the lake. I adjusted the trim and noticed that it would only intermittently work. Then, the trim gauge was jumping up and down like crazy and the trim stopped working all together. The gauge would still move around but not a sound when I hit the trim button. There are no blown fuses or anything. The rocker switch for the trim is on the throttle control. I'm wondering if anyone can tell me what probably happened. Is it possible that we damaged something by pushing the trim buttons while the negative was disconnected from the battery??? or is it possible that the switch is damaged? I know there are a lot of possibilities when it comes to electrical problems but I'm hoping someone who maybe had this same problem may know. I didn't have tools with me to take the throttle arm apart and check the switch. But anyways, What I'm really hoping is that I didn't damage something on the boat---because the trim was stuck all the way down and we had to drive through some really shallow stuff and the prop was in the muck for at least 100 yards---had no choice. So if anyone can please help me out---it'd be much appreciated. thank you Tom
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Hi Tom, I had this exact problem with a volvo penta I/O a few years back. We suspected and replaced the relay box ($130+) only to find that the problem still existed. It turned out that the position sensor in the outdrive had given up, and wasn't allowing the trim to operate. I imagine mercruiser has something similar. Anyway, this sensor measures the actual angle of the outdrive (for the trim gauge), and also keeps you from tilting it up to insane positions and flipping your boat. So when it failed open, the trim wouldn't work at all. The jumping trim gauge really points towards this being the same problem. So here's a really easy way to diagnose it, which came to me in a flash of inspiration about 5 hours too late: Somewhere on the throttle there will be a button that you press while pushing trim up, which lets you tilt the prop all the way up for trailering. This button overrides the sensor in question...so if the trim works while holding this button down, then the tilt sensor or the wiring to it definitely has a problem and you've saved quite a bit of time troubleshooting. If it still doesn't work, then it's time to dig out the voltmeter and an assistant and start probing. Anyway, that's how i diagnosed a very similar problem and hopefully that can save you some time. Also, I highly doubt that you did anything by using the switch without the negative attached. Hopefully all you did was pull a wiring harness slightly loose when reconnecting it, or something minor like that. But odds are great that the really expensive stuff (trim motor + hydraulics) is just fine. So breathe easier. Good luck! Henry.
